Major aspects of androgen action include significant effects on muscle growth as castration can markedly reduce the degree of experimentally produced skeletal muscle hypertrophy. Yet, androgen mechanisms on muscle development are unknown. The proposed studies have been designed to gain insights into androgen regulation by investigating in detail the time course of androgen responses in muscle and in blood of normal, castrated, and hypophusectomized animals undergoing compensatory muscle growth be ablation of a synergistic muscle. Another aim is to determine which androgenic hormone (testosterone or 5 Alpha -dihydrotestosterone) is primarily responsible for skeletal muscle growth. Androgen receptor capacity, binding affinity, and binding specificity (competition of receptor sites for androgens, estrogens, and glucocorticoids) in muscle, as well as blood levels of testosterone, dihydrotestosterone and testosterone-binding globulin sured in all studies to gain an understanding of androgen mechanisms in muscle. Recent studies from this laboratory have shown that endurance training can retard the muscle atrophy associated with glucocorticoid treatment. Therefore, a third aim will be to study androgen responses in atrophied muscle, resulting from glucocorticoids, and in muscle that is spared from atrophy by endurance training. A final aim will involve studying the possibility that elevated estrogen levels have an inhibitory effect on the development of hypertrophy. Long-term objectives include understanding the degradative processes associated with muscular-related disorders and dystrophies. Knowledge of the mechanisms contributint to muscle growth and to muscle atrophy can provide a biochemical basis fr counteracting and treating many muscular diseases. In addition, there is prevalent use of anabolic steroids (androgenic hormones) by athletes; thus, physiological and biochemical evidence of their effects are needed.